UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Amateur and Military Radio

Notices

Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Reply
 
Thread Tools
Old 25th May 2020, 11:29 pm   #141
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,457
Default Re: Superstar 6900N V6 - Fault with SSB mode

Here is the file, it was too large to fit on this site as a PDF in reasonable quality, so I have placed it on my website.
http://www.kbmuseum.org.uk/other_ser...vice_notes.pdf
Please help yourself and save to your PC.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is online now   Reply With Quote
Old 25th May 2020, 11:30 pm   #142
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,994
Default Re: Superstar 6900N V6 - Fault with SSB mode

Will do. Enjoy your radio.
SiriusHardware is online now   Reply With Quote
Old Yesterday, 12:35 am   #143
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,029
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by Mike. Watterson View Post
That's poor alignment advice. With the varied Vt., setting a particular gate volts is roulette. It's the current that counts.
I very strongly agree with these two statements, and I wouldn't have just called it poor.

You set whatever bias volts it takes to get the right current.

That's why two devices in parallel is a big no-no.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old Yesterday, 12:51 am   #144
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,994
Default Re: Superstar 6900N V6 - Fault with SSB mode

Sometimes the 'wrong' action leads to the right outcome, as seems to have happened here. In the ideal world we would know exactly the current required and would be able to advise how to go about adjusting the bias for that current.

Here, by a matter of luck, the bias voltage values crackle found online have turned out a reasonable match for his selected MOSFETs and put his radio into the zone where it works, and that was the outcome he was looking for. I think it best to leave it at that.
SiriusHardware is online now   Reply With Quote
Old Yesterday, 6:55 am   #145
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,457
Default Re: Superstar 6900N V6 - Fault with SSB mode

The problem I see is the circuit is not designed to enable you to check the current easily with standard domestic type DMM. I may be wrong but would the long leads to the meter in series with the supply to the OP pair cause problems.

OH, and nobody knows what the current should be set at.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is online now   Reply With Quote
Old Yesterday, 7:49 am   #146
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,994
Default Re: Superstar 6900N V6 - Fault with SSB mode

Mike W did cover the use of a meter to measure the current in some detail...

Quote:
For O/P or Driver current measuring:
Lift one end of the RF chokes that are wire links in ferrite tubes, judging from the photos. Tack 1nF ceramic there to a nearby earth with short leads. We don't want RF in the test meter! Twist test meter leads.
But you're right, we still don't know what exactly what the current should be anyway. As I said, arriving at your destination is usually more important than how you get there.

Wonder if the skip will be running today as well? Stuck at work, unfortunately, so I won't have much of a chance to find out.
SiriusHardware is online now   Reply With Quote
Old Yesterday, 8:13 am   #147
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,029
Default Re: Superstar 6900N V6 - Fault with SSB mode

If it was designed to have current sampling places, they would be decoupled so that test cables wouldn't create RF problems. You'd still need short thick cables because of the current involved and the need to not disturb things with voltage drop, so special cables would need to be made up. DMMs would be poor choices, most go loopy in the presence of RF power, so you'd be looking at something expensive from Fluke or HP which get carefully filtered - or just use a classic passive meter like an AVO!

As for the currents, it would be nice to know the design values, but from knowing the supply voltage and the output power, a pretty good guess can be made, and anyone using it will get a working transmitter. It's a matter of re-doing the design calculations, and not particularly difficult.

Trying to get other people to just set a gate voltage introduces the variation of the transistors in terms of Vgs versus Id. That transistor has a data sheet stating there is 2 volts of variation in the threshold voltage (2 to 4 volts) and that's only at 250uA in the drain. It gets worse at higher currents!

So, setting the bias by just the gate voltage is very easy to do and very easy to describe, but it will only work for a fraction of the transistors of that type out there. Some people will be left with too low bias and very little output power, others will take enough current to blow fuses. You've been lucky that a figure suggested to you worked for yours. Not everyone will be so lucky.

1) Designing something is one thing.
2) Designing it so that it can use any part that is within spec for that type of transistor is harder.
3) Designing it so that you can adjust what needs to be adjusted is harder still.
4) And finally, writing a procedure that other people can follow to set one up properly is a right sod of a job!

That radio is OK on 1) above.
It's a fail on 2) because it needs matched pairs. Can't be relied to work with randomly chosen transistors
as for 3) it's not too bad, there is a sequence which will get it there, given matched transistors. Not matched on a hand-held tester or a multimeter, but matched at high current as they will be used.
number 4) is a complete wipeout. There is no info available on how to select transistors and set it up.

So, some of us can work out what current to set it up to, and a process for setting it up, but there's as much work involved as there was in designing these aspects in the first place. There is a class of circuits which can only be fixed with any certainty by someone that could have designed it right in the first place!

Someone I used to know had a radio and TV repair business. He kept getting asked to repair guitar amplifiers in the early sixties. He lost his rag with some of the junk that came his way. "I could do better than that!" was his thought. So he did. His name was Matt Mathias, a jewish escapee from Nazi Germany. His amplifiers were branded "Matamp" but a guy with a boutique music shop in London wanted to buy lots with his own branding on them and bright orange tolex covering. So when you see famous groups fronting "ORANGE' speakers and amps, it all came out of the thought 'I could do better than that!'. Better build quality than Marshall, too. Matt was a nice guy, and the people who called in at his place were a who's who of rock and pop.

So, you learn more from coming across a piece of iffy design than you do from an honest part failure in a good design. Ironic, really.

I've been enjoying a few days of sporadic-E propagation on 50MHz. 20metres is coming alive, I think the next sunspot cycle is starting.... yippee!

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old Yesterday, 4:08 pm   #148
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,457
Default Re: Superstar 6900N V6 - Fault with SSB mode

Just before I put this radio back in its case I did a few more measurements.
I dont know if these will help.
The following currents were measured on the Volts Amps meter on my 12.3v bench PSU.

With the driver bias J4 off and the PA bias J3 on,
AM 1.37A
FM 1.87A

With J4 and J3 off the currents were
AM 0.8A
FM 1.09A

SSB current no audio TX with J4 & J3 on 0.5A
SSB current no audio TX with J4 & J3 off 0.48A hardly any difference.

Receive current 0.34A

Can the Output pair idle current be determined from the above readings?

Dont forget that these figures were all obtained with a bench supply of 12.3v.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is online now   Reply With Quote
Old Yesterday, 4:53 pm   #149
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,994
Default Re: Superstar 6900N V6 - Fault with SSB mode

This figure in particular:-

Quote:
SSB current no audio TX with J4 & J3 on 0.5A
SSB current no audio TX with J4 & J3 off 0.48A hardly any difference.
...makes it sound to me as though there is not much bias. I'll leave it up to others with better knowledge to take up the heavy lifting on this one.

A couple of questions to start things off, though. Which FETs from your original 'tested' list did you fit?

Can you also state the maximum power that the radio should have on AM / FM / SSB according to the user manual / specifications?
SiriusHardware is online now   Reply With Quote
Old Yesterday, 5:47 pm   #150
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,029
Default Re: Superstar 6900N V6 - Fault with SSB mode

In lieu of a two-tone audio intermodulation test into a spectrum analyser, getting reports of how much distortion and adjacent channel splatter there is on SSB might be the only check

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old Yesterday, 7:21 pm   #151
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,457
Default Re: Superstar 6900N V6 - Fault with SSB mode

Here are the specs from the CRT SS6900N v6 user manual.

Mike
Attached Thumbnails
Click image for larger version

Name:	SS6900n V6 user manual0017.jpg
Views:	8
Size:	126.3 KB
ID:	206857  
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is online now   Reply With Quote
Old Yesterday, 7:38 pm   #152
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,994
Default Re: Superstar 6900N V6 - Fault with SSB mode

OK, so

12W AM
30W SSB
40W FM

I think Mike W and David both felt that it might be possible to estimate the likely required driver and output bias current if they knew these figures.

And sorry, do you know which pair of FETs you fitted? It might be useful to know what the VT figure was for them.
SiriusHardware is online now   Reply With Quote
Old Yesterday, 7:44 pm   #153
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,457
Default Re: Superstar 6900N V6 - Fault with SSB mode

Although the radio was capable of achieving those power readings, everyone you speak to says it is not a good idea, for long at least not on FM.

Here is the full user manual, with a few comments by me. http://www.kbmuseum.org.uk/ss6900nv6...ser_manual.pdf
You may notice on the second page it says duty cycle TX 10%
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is online now   Reply With Quote
Old Yesterday, 7:52 pm   #154
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,994
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by crackle View Post
You may notice on the second page it says duty cycle TX 10%
I can't imagine it lasting long on 10m, then (we amateurs are notorious wafflers).

Even if rarely used on full power it needs to be set up as though it is going to be so that it works right across the full variable power range. That's why I asked what the maximum figures were.

I had a look at the CRS SS6900 on Knights and I notice they offer it either in a full power version (yours) or a power-limited version (still quite powerful) in which the transmitter is dialled back a bit, probably to enhance its own survival.
SiriusHardware is online now   Reply With Quote
Old Today, 6:54 am   #155
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,457
Default Re: Superstar 6900N V6 - Fault with SSB mode

Sorry I forgot to mention, I fitted the two IRF520s that you thought were best matched, E & H
Incidentally the 2 that were in the radio when it was not working on SSB were A & B. I don't know why I chose such a badly matched pair, I suppose I just didn't realise the significance.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is online now   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 9:46 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.